The most commonly used electrolytes are fluid liquids which comprise solutions in a liquid solvent of solute ionic species. Such fluid liquid electrolytes, on incorporation into a galvanic cell, permit migration of ions between the electrodes of the cell and, as a consequence, the provision of electric energy to a closed external circuit. Despite their widespread use, such electrolytes nonetheless suffer from several disadvantages. They are often corrosive, leading to leakage from cells and they do not provide a firm barrier between the electrodes when required to assist in stabilizing the inter-electrode distance and in preventing physical loss of electrode material from the electrode surface.
In order, in part, to overcome the disadvantages inherent in fluid liquid electrolytes, particularly in relation to galvanic cells, considerable effort has been expended in attempts to provide solid or highly viscous polymeric electrolytes which contain salts which display mobility, under appropriate conditions, of at least some of the ionic species present. The solid polymeric electrolytes are capable of acting in thin film form as electrode separators and in solid-state cells can deform while maintaining good contact with the electrodes, thus minimizing problems arising from mechanical strain arising either from mechanical stresses during use or volume changes during the charge/discharge cycle. A particular area of importance is in cells that do not depend upon water as a component of the electrolyte, such as lithium cells where water and other materials capable of reacting with lithium are undesirable. The potential uses for such materials are not limited to batteries but include, inter alia, sensor devices and thermoelectric energy convectors.
A prominent polymeric material for this purpose has been poly(ethylene oxide) (PEO), in which certain salts are soluble and can form complexes. The electrical and mechanical properties of such polymer electrolyte materials, although encouraging, require further enhancement before commercialisation can be envisaged. Improvements in the properties have been obtained using graft copolymers in which short poly(ethylene oxide) chains are present as pendant units attached to a long main chain. Such materials have been described In GB-A-2161488. Another means of improving the mechanical properties is to use block copolymers in which short poly(ethylene oxide) chains alternate with other units such as polysiloxane. Yet another means is to cross-link a poly(ethylene oxide) with an epoxy compound. In each case the polymer electrolyte contains a suitable salt complexed with the polymer to provide the ionic species required for conductivity. In all these cases the conductivities reported at 25° C. or at room temperature are at best about 10−4 Siemens per cm. These values are an order of magnitude less than a commonly cited target for commercial realization of 10−3 Siemens per cm.
It is also possible to provide polymer electrolytes which consist of a mixture of a polymer, preferably of high molecular weight, with a compound of low molecular weight that is a solvent for the polymer in the range of temperatures in which the electrolyte is to be used, together with an appropriate salt that is soluble in the polymer and in the compound of low molecular weight. For example, as disclosed in GB-A-2212504 and 2216132, polymer electrolytes consisting of poly-N,N-dimethylacrylamide or closely related poly-N-substituted acrylamide of high molecular weight plasticized with dimethylacetamide together with lithium trifluoromethane sulphonate (lithium triflate) as the salt component have been evaluated and found to exhibit good conductivities together with good mechanical properties. These polymer electrolytes are gel-like in character, but the compound of low molecular weight must not exceed a certain limiting concentration above which the system loses its gel-like character and begins to flow. The ionic conductivity is higher at the higher concentrations of the compound of low molecular weight, but the material becomes increasingly more flexible. Conductivities of 7×10−3 S cm1 at 20° C. are obtainable but this requires at least 60% or more of the low molecular weight compound and at this level the mechanical properties are poor. It has proved possible by cross-linking the polymer to improve the mechanical properties to a useful level with as much as 80% of the low molecular weight compound present, and thus to obtain conductivities at 20° C. exceeding 10−3 Scm1. These products may prove of commercial interest, but the process for making the cross-linked polymer electrolyte film is somewhat complex for convenient incorporation into a process for cell manufacture.
However it is well established that with electrolytes containing propylene carbonate (PC) as the liquid component in whole or in part, for most anode electrode systems, particularly those comprising graphite, exfoliation of the electrode occurs. This exfoliation continues throughout the charge-discharge process, resulting in extensive capacity loss on the first cycle and progressive capacity loss in subsequent cycles. For liquid cells the industry therefore favours a combination of aprotic organic solvents other than PC, eg. EC, DEE, EMC etc. many of which are much more volatile than PC. These can be used in cell manufacture which takes place at ambient temperatures.
The use of volatile organic liquids is generally problematic for the formation of polymer gel electrolytes (PGE) by extrusion lamination at high temperatures. High boiling point solvents such as ethylene carbonate (EC) (bp 244° C.) and PC (bp 240° C.) are required in the major proportion for the formation of the PGEs. But EC has a high melting point (36° C.) and a non-trivial proportion of PC (mp −55° C.) is required to prevent the gel from freezing at ambient temperatures. EC and PC are benign non-toxic materials which are easy to handle. Other higher boiling point solvents from which PGEs could be formed (eg. Dimethyl formamide (DMF) or NMP) are more hazardous and often toxic. Gels composed of EC and more volatile components are generally of poorer quality mechanically than those with substantial quantities of PC. PGEs formed with PC either alone or in conjunction with EC are of the highest quality.
Thus at present there is a balance to be obtained between the aprotic solvents used, their ratio and the choice of anode composition and construction.
There are anode compositions incorporating different forms of graphite which can be used with PC with minimal exfoliation and capacity loss, but their use is restrictive. Hitherto there has not been an effective additive agent in surpressing this exfoliation and loss of capacity. Such an agent, working on all graphitic anodic materials would greatly enhance the PGE extrusion-lamination process with the unrestricted use of PC.
More recently a new lithium salt, lithium bis(oxalate)borate (LiBOB) has become available and has been proposed for use with graphitic anodes. By the addition of LiBOB at as low as 5% molar, exfoliation in the graphitic anode has been shown to be effectively prevented.
It is now proposed that the addition of LiBOB to the polymer gel electrolyte itself, generally in conjunction with the extrusion lamination procedure, will greatly extend the range of materials available for the fabrication of electrical components (particularly galvanic cells) and greatly enhance the performance of these components.